This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. This subproject involves the development and construction of an ESR microscope operating at pulsed mode. The ESR microscope is conceptually similar to the well known Magnetic Resonance Imaging (MRI) system. The main differences are with the type of spins used for the measurements (electron vs. protons in MRI), and the image resolution (microns vs. mm). Consequently, most biologically-related samples must be doped or labelled with stable radicals to facilitate the ESR measurement (similar to dyes in optical microscopy). Furthermore, the samples must be with a typical size of no more than 1-2 mm. When combined with optical microscopy, the pulsed ESR microscope can be a valuable tool in bio-science. Spatially resolved parameters such as diffusion tensor, O2 concentration, relaxation times (T1, T2), and the ESR lineshape can reveal a wealth of information in model and live systems, inaccessible by optical methods. In the past attempts has been made to combine NMR and optical microscopy, but with limited success due to the limited resolution of NMR (~10 microns) and its high cost. The ESR microscope, which is much more sensitive, should achieve magnetic resonance images with resolution comparable to optical method (~ 1 micron), and at much lower cost (simple electromagnet vs. large superconducting magnetic in NMR microscopy).We have constructed an initial prototype of the pulsed ESR microscope. The system is made of a home-built 6-17 GHz pulsed microwave bridge, a pulsed gradient driver, a computer for system control and data acquisition, and a pulsed imaging probe (resonator+ gradient coils). Three dimensional ESR images with a resolution of 3x3x8 microns were demonstrated at 16 GHz. Further progress to higher frequencies 9up to ~60 GHz, would facilitate the required sub-micron image resolution.